Yes, the Zen tradition, in all its forms, requires a teacher-student relationship.

I think there are many true Zen teachers, at least as it appears to me in Northern California.

“Enlightenment means to see what harm you are involved in and to renounce it.” David Brazier, The New Buddhism

“The most straightforward advice on awakening enlightened mind is this: practice not causing harm to anyone—yourself or others—and every day, do what you can to be helpful.” Pema Chodron, “What to Do When the Going Gets Rough”

"If you can become enlightened yourself, don’t rely on external seeking — don’t think I’m saying you can only attain emancipation through [the help of] a spiritual compatriot other than yourself. This is not the case! Why? Within your own minds there is a spiritual compatriot [who will help you] become enlightened by yourself! If you activate the false and deluded, you will become all mixed up with false thoughts. Although some external spiritual compatriots may be teachers, they cannot save you. If you activate the correct and true and contemplate with prajñā, in a single instant [all your] false thoughts will be completely eradicated. If you recognize the self-nature, with a single [experience of] enlightenment you will attain the stage of buddhahood."
(Platform Sutra, ch 2, BDK ed, p 33)

And Dahui,

"I just believe that this matter can’t be transmitted and can’t be learned. You must realize on your own, awaken on your own, affirm on your own, stop-to-rest on your own, and then you will for the first time get to the end of things."
(Letters of Dahui, p 89)

But even if it is maintained that there is no Zen without a teacher, it is taught that there is no Zen without the pure precepts of the renunciates.

The Buddha taught,

"Therefore, Ānanda, one who practices entering samādhi while practicing meditation in stillness without renouncing sexual activity is like one who cooks sand in the hope that it will turn into rice. A hundred thousand eons might pass and it would still be nothing but hot sand, since it wasn’t rice to begin with. It was merely sand.
In seeking the wondrous enlightenment of the Buddha while you still have sexual desire, you may gain some understanding of that wondrous enlightenment, but that understanding will be rooted in sexual desire. If the basis of your understanding is sexual desire, you will continually be reborn among the three lowly destinies, bound to the cycle of death and rebirth with no hope of escape. Then how will you find your way to practice and realization of the Thus-Come Ones’ nirvana?"
(Surangama Sutra, ch 7.1, new BTTS translation, p 265)

And Eisai,

"The Buddha’s teachings regard keeping precepts as preceding everything else. If anyone breaks moral precepts regulated by the Buddha, and calls himself a child of the Buddha, it can be compared to a subject who won’t obey the monarch’s orders but who [still] calls himself a subject of the monarch."
(Treatise on Letting Zen Flourish, in Zen Texts, BDK ed, p 125)

And Dogen,

"The lifeblood of the buddhas and the patriarchs is nothing other than their leaving family life and receiving the precepts. Someone who has not left family life is never a Buddhist patriarch. To see the buddhas and to see the patriarchs is to leave family life and to receive the precepts."
(Shukke, in SBGZ, vol 4, BDK ed, p 148)

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

... "I just believe that this matter can’t be transmitted and can’t be learned. You must realize on your own, awaken on your own, affirm on your own, stop-to-rest on your own, and then you will for the first time get to the end of things."
(Letters of Dahui, p 89) ...

... and in order to do so, you have to receive the intructions of a teacher to be able to do it, right?

Life is great and death has to be just as great as life.
- Mike Tyson
People not only don't know what's happening to them, they don't even know that they don't know.
- Noam Chomsky

... "I just believe that this matter can’t be transmitted and can’t be learned. You must realize on your own, awaken on your own, affirm on your own, stop-to-rest on your own, and then you will for the first time get to the end of things."
(Letters of Dahui, p 89) ...

... and in order to do so, you have to receive the intructions of a teacher to be able to do it, right?

there is no doubt about it.. those who claim it opposite way or say that there are different ways etc. are simply cheating themsleves and misguiding others.

What counts as Zen practice?
If it is non-thought, then if one can do it, there is nothing more to explain, and if one cannot do it, one is not yet practising.
If it is something other than non-thought, but rather various methods to achieve it, then it is no different from the gradual path that is explained in all the scriptures and treatises.

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

... and in order to do so, you have to receive the intructions of a teacher to be able to do it, right?

Dahui's letters are filled with instructions. But of course it might help to hear the same repeated from the high seat.

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

so, there is a difference between reading instructions in a book and receiving instructions directly from a teacher, isn't it!?

Is there a difference between writing and speaking?

"Since they maintain they have no need of written words, they should not speak either, because written words are merely the marks of spoken language. They also maintain that the direct way cannot be established by written words, and yet these two words, ‘not established’ are themselves written."
(Platform Sutra, ch 10, tr BTTS)

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

... "I just believe that this matter can’t be transmitted and can’t be learned. You must realize on your own, awaken on your own, affirm on your own, stop-to-rest on your own, and then you will for the first time get to the end of things."
(Letters of Dahui, p 89) ...

... and in order to do so, you have to receive the intructions of a teacher to be able to do it, right?

The instructions I once received from my teacher were "Nothing that I can say can help you". I found that to be the most helpful thing he's ever said.

One should not kill any living being, nor cause it to be killed, nor should one incite any other to kill. Do never injure any being, whether strong or weak, in this entire universe!

so, there is a difference between reading instructions in a book and receiving instructions directly from a teacher, isn't it!?

Is there a difference between writing and speaking?

"Since they maintain they have no need of written words, they should not speak either, because written words are merely the marks of spoken language. They also maintain that the direct way cannot be established by written words, and yet these two words, ‘not established’ are themselves written."
(Platform Sutra, ch 10, tr BTTS)

since you quoted Dogen before, here is a quote from Dogen's Bendowa (emphasis is mine):

You must clearly understand that the wondrous dharma of the seven buddhas manifests its essential meaning and is received and upheld only when a practitioner matches the mind, actualizes awakening, and follows and receives the true transmission of a master who embodies the Way and clarifies the mind. This [wondrous dharma] cannot be fully understood by a teacher who only studies words. Therefore, immediately cease this skepticism, practice the way of zazen under the guidance of a true teacher, and fully actualize the jijuyu zanmai of the buddhas.

... Just seek the guidance of a true teacher.

and one quote from his Gakudo Yojin-shu:

... You should seek a true teacher to practice Zen and study the way. ... Until you have a true teacher, it is better not to study. ... To understand the dharma and attain the way can only be the result of studying with a teacher.

Life is great and death has to be just as great as life.
- Mike Tyson
People not only don't know what's happening to them, they don't even know that they don't know.
- Noam Chomsky

Astus; Thank you for the quote from the Platform Sutra. It’s been a few years since I studied it.

It occurs to me that some of the objections raised are based on different understandings of “Zen” as a tradition, as a practice, and as enlightenment.

I defer to Zen teachers to define the “Zen tradition” and most (all?) Zen teachers hold that the tradition requires a living, face-to-face student-teacher relationship. So, a ‘true Zen student’ has a Zen teacher, which is why I consider myself a ‘student of Zen’ and not a Zen student.

It seems that most Zen practices, with the notable exception of traditional Rinzai koan practice, can be practiced without a teacher. Of course, as you noted, one gets guidance from reading Sutras and the writings of Zen teachers, perhaps listening to podcasts, or occasionally sitting with a Zen teacher.

And if “Zen” is used to refer to enlightenment, no tradition owns that.

“Enlightenment means to see what harm you are involved in and to renounce it.” David Brazier, The New Buddhism

“The most straightforward advice on awakening enlightened mind is this: practice not causing harm to anyone—yourself or others—and every day, do what you can to be helpful.” Pema Chodron, “What to Do When the Going Gets Rough”

That means only one thing in Dogen's writing, himself. That is because he denies the realisation of all Buddhist teachers who brought the Dharma from China before him, and he viewed only his teacher as the sole upholder of not simply Zen but the whole Buddhadharma. In other words, what Dogen claims (or rather propagates) is that unless you become his disciple, you cannot attain liberation.

To understand the dharma and attain the way can only be the result of studying with a teacher.

Any reason why that would be so?

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

To understand the dharma and attain the way can only be the result of studying with a teacher.

Any reason why that would be so?

Because without a teacher one must rely on one’s own faulty awareness as one’s guide. Unawareness will always interpret Dharma as something other than what it is.

1.The problem isn’t ‘ignorance’. The problem is the mind you have right now. (H.H. Karmapa XVII @NYC 2/4/18)2. I support Mingyur R and HHDL in their positions against lama abuse.3. Student: Lama, I thought I might die but then I realized that the 3 Jewels would protect me. Lama: Even If you had died the 3 Jewels would still have protected you. (DW post by Fortyeightvows)

I defer to Zen teachers to define the “Zen tradition” and most (all?) Zen teachers hold that the tradition requires a living, face-to-face student-teacher relationship. So, a ‘true Zen student’ has a Zen teacher, which is why I consider myself a ‘student of Zen’ and not a Zen student.

Every skill and science has its teachers, because on the one hand there are people who like to share their knowledge, and there are people who like to listen to others. This is nothing unique in Zen. But while a discussion on history means talking about historical events, apparently lots of discussion about Zen is about the people who teach and who study it. In that way Zen looks more like the shallow analysis of modern politics where people consider only the individual politicians and their followers, instead of politics on the social, economic, and the philosophical level. Probably the reason for that is the idea that Zen is some undefinable practice that cannot be spoken of, hence instead of clarifying what is what, it is left to those who are assumed to be the experts in this mostly unknown matter. At the same time, there is the fear that unless one attends a specialist, one will fall into unfounded conceptual constructs and misinterpret everything. But isn't it because of leaving Zen a vague something that it is so easy to be trapped into wrong views about it? Just because there are hundreds and thousands of reliable sources to study mathematics, it does not mean people no longer want to learn it from the experts of the field, and at the same time there is little confusion about the difference in expertise of a freshman, a secondary school teacher, and a Fields Medalist. Similarly, there are village Zen priests, there are abbots, there are highly ranked masters, and as for genuinely enlightened teachers, that is rather a matter of personal opinion. But none of that actually informs us what Zen is, just as living with a mathematician does not magically make one understand mathematics.

practiced without a teacher

Is practice a communal activity, or two-person activity? If not, and it is an individual effort, then no practice happens with a teacher. A teacher can explain and advise, but can neither show it, nor give it.

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

Because without a teacher one must rely on one’s own faulty awareness as one’s guide. Unawareness will always interpret Dharma as something other than what it is.

All practitioner must develop for oneself correct mindfulness (samyaksmrti) and wise attention (yonisomanaskara). But I think you mean right view, i.e. correctly knowing and understanding the Dharma, what is obtained by learning, and learning happens through listening to Dharma speeches and reading Dharma books. Of course, without learning there is little chance of arriving at correct view, and without correct view there is no correct understanding and correct cultivation.

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

Because without a teacher one must rely on one’s own faulty awareness as one’s guide. Unawareness will always interpret Dharma as something other than what it is.

All practitioner must develop for oneself correct mindfulness (samyaksmrti) and wise attention (yonisomanaskara). But I think you mean right view, i.e. correctly knowing and understanding the Dharma, what is obtained by learning, and learning happens through listening to Dharma speeches and reading Dharma books. Of course, without learning there is little chance of arriving at correct view, and without correct view there is no correct understanding and correct cultivation.

Of course, no doubt about that, but it is very wise to check back with the teacher in order to not go astray. This is the danger of doing it by oneself alone, as I see it. It is good to check ones understanding with a teacher, a real person, to avoid fooling oneself.

Life is great and death has to be just as great as life.
- Mike Tyson
People not only don't know what's happening to them, they don't even know that they don't know.
- Noam Chomsky

Of course, no doubt about that, but it is very wise to check back with the teacher in order to not go astray. This is the danger of doing it by oneself alone, as I see it. It is good to check ones understanding with a teacher, a real person, to avoid fooling oneself.

According to Shengyan the primary factor is that a teacher should have a correct view, and in order to be able to tell that, the disciple must have knowledge of the Dharma. So in the end it always comes down to matching view with what is taught in the scriptures. As it's said in the Mahaparinibbana Sutta:

"If they’re not included in the discourses or found in the Vinaya, you should draw the conclusion: ‘Clearly this is not the word of the Buddha. It has not been correctly memorized by those senior mendicants.’ And so you should reject it. If they are included in the discourses and found in the Vinaya, you should draw the conclusion: ‘Clearly this is the word of the Buddha. It has been correctly memorized by those senior mendicants.’ You should remember it."

1Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?2If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.3Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.4With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.

There is also the danger of selecting a false teacher, which is ruinous for the teacher and the student alike. Given this, and all of the warnings about false teachers in Zen texts, simply diving into modern "Zen" teachings without knowing (or maybe caring) how to discern false teachers is just as bad of an idea as practicing on your own.